Fix MBL zigzag calls

Marlin/Marlin_main.cpp

       return;
     }
 
-    int ix, iy;
+    int8_t ix, iy;
     float z;
 
     switch (state) {

Marlin/mesh_bed_leveling.h

     }
 
     void set_zigzag_z(int8_t index, float z) {
-      int ix, iy;
+      int8_t ix, iy;
       zigzag(index, ix, iy);
       set_z(ix, iy, z);
     }

Marlin/ultralcd.cpp

     lcd_goto_menu(_lcd_level_bed_moving);
 
     // _mbl_goto_xy runs the menu loop until the move is done
-    int ix, iy;
+    int8_t ix, iy;
     mbl.zigzag(_lcd_level_bed_position, ix, iy);
     _mbl_goto_xy(mbl.get_x(ix), mbl.get_y(iy));